Among the 17 mycoviruses which have been physically characterized, there are two which code for specific toxins lethal to the non-infected strains of the same fungi. These are the mycoviruses Saccharomyces cerevisiae and Ustilago maydis. These viruses have many similar properties, including segmented dsRNA genomes which may be grouped into classes of heavy (4-6 kbp), medium (0.7-1.4 kbp) and light (less than 0.35 kbp) segments. In both viruses the heavy segments code for capsid production and the medium segments code for toxins. There are three strains of the Ustilago virus which are distinguishable by the number of genomic segments of each class and slight differences in specificity of the toxins. In this project relationships of the dsRNA segments within and among the three strains will be determined using heterodupliex and hybridization procedures. The ability of each segment to be transcribed to mRNA and the translation product of representative dsRNA segments will be determined. Specific dsRNA segments will be cloned in E. coli for the study of amplified gene products and sequence analysis. These studies will provide a better understanding of how toxins coded by viruses interrupt or relate to host cell metabolism. Such basic knowledge will be used in the control of pathogenic fungi or the control of specific toxins secreted by fungi which result in food poisoning.